A large portion of the materials which we walk on, e.g. insoles of shoes, indoor carpeting, outdoor carpeting, floor mats, etc., have a basic construction which has remained substantially the same for a number of years. In most instances, such materials consist essentially of a top layer of fibrous cloth or fabric material which touches the foot, and a second layer of backing material connected to the top layer which may or may not have cushioning properties. Despite improvements in certain aspects of these types of products such as wear life and comfort, little or no attention has been paid to characteristics such as moisture-absorbency, slip-resistance or thermal conductivity.
For example, in the design of insoles for shoes, and particularly insoles for athletic shoes or other active wear articles of footwear, an effort has been made to improve the cushioning characteristics of sheet material utilized to fabricate such insoles to enhance comfort. But the configuration and surface characteristics of the sheet material for such insoles have been ignored as they relate to (1) the ability of the insole to maintain the foot and sock insulated from the sole of the article of footwear; (2) the extent to which the foot and sock are permitted to move within the article of footwear; and (3) the ability of the insole to maintain the wearer's sock substantially dry.
One problem with the sheet materials employed to fabricate insoles for active wear footwear involves a failure to control the motion of the sock of the wearer relative to the insole and/or the motion of the foot of the wearer with respect to the sock. This affects both the comfort and performance of the shoe. For example, certain activities such as the play of tennis on clay courts and soccer on grass result in substantial movement of the shoe with respect to the playing surface. In these types of activities, it is desirable to limit the movement of the foot and sock with respect to the insole of the article of footwear for added comfort and to optimize the performance of the footwear. On the other hand, comfort and performance dictate that the foot and sock be permitted more movement within articles of footwear intended for use on high friction surfaces such as lacquered hardwood floors or artificial playing fields wherein limited movement of the article of footwear relative to the playing surface is permitted, and therefore relatively high shear forces are transmitted from the shoe to the foot.
Prior insoles can generally be divided into two categories, depending upon the type of sheet material employed, both of which fail to take into account the movement of the foot and/or sock within the article of footwear and the type of surface on which the footwear is utilized. In some designs, the top surface of the sheet material from which such insoles are made is formed of a tacky or sticky sheet material, or a material which becomes relatively tacky when exposed to moisture from the foot. Insoles made with sheet material of this type exhibit a higher coefficient of friction than the coefficient of friction of the skin of the foot. As a result, the magnitude of the frictional engagement between the sock and the sheet material at the top of the insole is greater than the magnitude of the frictional engagement between the foot and sock. Articles of footwear provided with insoles having this type of sheet material have been found to create blisters on the foot during use because the foot is allowed to move within the sock in response to the application of a shear force, i.e., a front-to-rear, side-to-side and/or rotational foot motion, while the sock is held in an essentially fixed position atop the insole. The rubbing motion of the foot within the sock can create severe blistering and discomfort, particularly in activities such as basketball, racquetball, aerobics and the like played on hardwood floor which permit limited motion of the shoe therealong.
Another general category of insole designs employs sheet material formed with a rubber or foam bottom layer which is covered by an overlayer of cloth or synthetic sheet material having a relatively slippery or slick surface with a much lower coefficient of friction compared to that of the skin. Insoles made with sheet material of this type help avoid the blistering problem because the foot and sock can move as a unit relative to the slippery top layer of the insole, instead of the foot moving within the sock. But the problem with these insoles is that movement of the sock and foot of the wearer is often completely unrestricted by the material forming the top layer thereof, and the toes are permitted to violently slide into the toe portion of the article of footwear causing bruising or even fractures. In addition, undue movement of the foot and sock gives the wearer a feeling of lack of control of the footwear, particularly in activities where the footwear readily slides along the playing surface.
Both types of sheet materials described above also suffer from a complete failure to assist in maintaining the foot and sock of the wearer dry. Whether the top, foot-engaging layer of the sheet material has a high or low coefficient of friction, such layers have no capability of wicking or otherwise removing moisture from the interface with the foot or sock. As a result, the insoles made from such sheet materials have an uncomfortable wet or damp feel, particularly when used in articles of footwear intended for athletic activities or other active wear.
Problems with slip-resistance, moisture-absorbency and thermal insulation are also prevalent in sheet materials intended for products other than insoles, e.g. indoor-outdoor carpeting; floor coverings for boats, campers, swimming pool decks, etc; floor mats, gloves, grips for tools and the like, and other items. In many products of this type, the surface which comes into contact with the foot or hands has limited slip resistance and can become particularly slippery and hazardous when wet. Additionally, failure to wick away moisture from the surface can further reduce slip-resistance and create discomfort to one using or wearing such sheet material. Further, sheet materials used for different types of floor coverings or the like often provide only limited thermal insulation to protect the feet from the heat or cold of the surface upon which the sheet material rests.
These problems have been addressed in U.S. Pat. Nos. 4,893,418 and 4,925,724, owned by the assignee of this invention. The sheet material disclosed in the U.S. Pat. No. 4,925,724 patent comprises a bottom layer formed of a cushioning material such as rubber or foamed plastic having an upper surface and a lower surface. The top layer of the sheet material is formed of a non-absorbent, thermally non-conductive thermoplastic material having a plurality of apertures which define intersecting columns and rows of thermoplastic strands or wall sections. The top layer is at least partially embedded in the bottom, cushioning layer so that a portion of the top layer extends beneath the upper surface of the bottom layer and the cushioning material forming the bottom layer at least partially enters the apertures in the top layer.
As discussed in U.S. Pat. No. 4,925,724, the thermoplastic material forming the top layer of the sheet material can be varied to alter the coefficient of friction, as desired. When used to manufacture an insole, such sheet material exhibits frictional characteristics which are effective to control the movement of the foot and sock within an article of footwear. In particular, the coefficient of friction of the apertured top layer of the sheet material is chosen such that the magnitude of the frictional engagement between the sock and such top layer is less than the magnitude of the frictional engagement between the foot and sock. This is true whether the sheet material is wet or dry. As a result, the foot and sock move together as a unit with respect to the top layer of an insole formed with such sheet material in response to the application of a shear force to the foot, thus preventing the blistering problem caused by prior insoles mentioned above. Additionally, the coefficient of friction of such top layer of the sheet material is chosen to provide at least some slip resistance where the article of footwear is intended for use on playing surfaces with a high coefficient of friction (hardwood floors, artificial fields), and a somewhat greater slip-resistance where the article of footwear is intended for use on surfaces having a lower coefficient of friction (e.g., clay tennis courts, grass fields, etc.).
While the sheet material disclosed in U.S. Pat. No. 4,925,724 provides a number of advantages, it has been found that some potential problems can arise with the use of such material to form insoles for articles of footwear, indoor-outdoor carpeting, floor mats and other items. As mentioned above, the apertures in the top layer of the sheet material form a matrix of interconnected wall sections, such as squares, triangles or the like. These wall sections are on the order of about 0.6 millimeters in thickness and about 0.5 millimeters in width. The thermoplastic material utilized for form this relatively thin apertured top layer exhibits good strength in compression, but is comparatively weak in shear. As a result, shear forces created by front-to-back, side-to-side and/or rotational motion along the apertured top layer has a tendency to stretch, pull or otherwise move the wall sections of the top layer relative to one another. The resilient cushioning material within which the apertured top layer is embedded offers substantially no resistance to the application of such shear forces and thus readily permits such relative motion of the wall sections. The apertured top layer is therefore subject to tearing or ripping of its wall sections, and the cushioning material beneath can become worn and break down as the apertured top layer moves therealong.
Another potential limitation of the sheet material disclosed in U.S. Pat. No. 4,925,724 is that moisture from the foot and sock can collect along the apertured top layer because little or ineffective wicking and/or absorption of such moisture takes place within the cushioning layer beneath. Although some types of open cell foam materials can be utilized to form the cushioning layer affixed to the apertured top layer, such foam materials provide only a limited degree of moisture absorption and little or no wicking or channeling away of moisture from the top layer. Other types of foam materials, such as closed cell foams, provide essentially no absorption or wicking capability whatsoever.
Another potential problem with the use of the sheet material disclosed in U.S. Pat. No. 4,925,724 in certain types of applications is that the apertured top layer can become delaminated from the cushioning layer. As disclosed in such patent, the apertured top layer and cushioning layer are interconnected by introducing the top layer onto the cushioning layer when it is in a "foamed" state, i.e. wherein the material has the consistency of whipped cream or the like before it is cured to form a solid sheet. Alternatively, the apertured top layer can be affixed to cushioning materials such as polyurethane which is liquid when initially combined with the top layer and thereafter cures to form a solid layer. In either case, the only connection between the apertured top layer and cushioning layer is the extent of surface contact between the cushioning material and the bottom and sides of the wall sections of the apertured top layer. This is a relatively small surface area. Additionally, the wall sections are made relatively smooth to provide comfort when contacted by the foot, which further increases the difficulty of obtaining a secure bond between the top layer and cushioning layer sufficient to avoid delamination.